1. Field of the Invention
The present invention relates to an original feeding apparatus and an image forming system having such original feeding apparatus, and more particularly, it relates to an image forming system, such as a copying machine, a laser beam printer and the like, having an original feeding apparatus for feeding a sheet original to a predetermined position such as an image reading station and for resting the sheet original thereon.
2. Related Background Art
In the past, as shown in FIGS. 10 and 21, original feeding apparatuses were divided into two groups.
FIG. 10 shows an original feeding apparatus of switch-back type wherein a sheet original is fed from a sheet tray 4 to an image reading station (platen 3) from an end B of the glass platen 3 and the sheet original is rested on the platen at a predetermined position. Thereafter, an image reading portion (optical system) of an image forming system is shifted to read the image, and, then, the sheet original is discharged from the end B of the glass platen and is stacked on the sheet tray again.
FIG. 21 shows an original feeding apparatus of a type wherein, similar to FIG. 10, a sheet original is fed to an image reading station (platen 3) from an end B of the glass platen 3 and the sheet original is rested on the platen at a predetermined position. After the image reading operation, the sheet original is discharged from the end B of the glass platen (switch-back type) or the sheet original is discharged from an opposite end C of the glass platen into a sheet path communicating with the sheet tray (closed-loop type). The closed-loop type or the switch-back type is selected in accordance with a size of the sheet original. In the original feeding apparatus of closed-loop type, after the image reading operation, the sheet tray was positioned horizontally or slightly inclined upwardly to facilitate the re-stacking of the sheet originals on the sheet tray 4.
In an original-through scanning mode in the conventional image forming system as shown in FIGS. 10 and 11, after the sheet original P from the sheet tray 4 has been rested on the glass platen 3 at any position, an optical system 213 was fixed, and the image reading was effected while discharging the sheet original P in a direction A.
However, there were the following drawbacks in performing the image formation by the aforementioned conventional techniques.
In the switch-back type shown in FIG. 10, since the next sheet original P was rested on the platen 3 after the first sheet original P has been discharged from the platen, a time (referred to as "original exchange time" hereinafter) required to discharge the first sheet original from the platen after the image reading operation and to set the next sheet original to be treated on the platen was long (corresponding to a time for feeding two sheet originals). In a high speed image forming system, since a copy sheet-to-copy sheet time (for example, a value obtained by dividing a distance between a trailing end of the first sheet and a leading end of the second sheet by a process speed) becomes shorter as the system is operated at higher speed, when "1 to 1" image formation (one image is formed on one sheet) is effected, if the sheet original exchange time is not smaller than the copy sheet-to-copy sheet time, it is impossible to make the productivity in the "1 to 1" image formation in the image forming system 100%. In the original feeding apparatus of the type shown in FIG. 10, since the sheet original exchange time was long, the productivity of 100% could not be achieved.
On the other hand, in the original feeding apparatus of the type shown in FIG. 21, although the sheet original can be fed by the switch-back mode as similar to FIG. 10 regarding a large size (for example, A3 size) sheet original, a half size (for example, A4 size and the like) sheet original is treated by the closed-loop mode. As shown in FIG. 22, in the closed-loop path, after the image reading operation regarding the first sheet original, the next sheet original is rested on the image reading station. In this closed loop type, since the sheet original exchange time is relatively short (a time required for feeding the sheet original by a distance corresponding to one sheet original plus a gap between two sheet originals), it is possible to exchange the sheet originals faster than the switch-back type shown in FIG. 10; but, if a further high speed operation is desired, since the feeding distance for the sheet original is already fixed or determined, the feeding speed must be increased. However, if the feeding speed is increased, the control for the stop position with high accuracy will become difficult, the damage of the sheet original will be increased when the sheet original is jammed, and the system will become large-sized and expensive and great noise will be generated because a large-sized motor must be for the higher speed operation.
To avoid this, in order to perform the high speed image formation without using the high speed sheet original feeding, it is known to fix the image reading portion of the image forming system and to read the image while shifting the sheet original. When the original-through scanning is effected in the types shown in FIGS. 10 and 21, unless the image reading is performed while the sheet original is being fed from the end C to the end B of the glass platen, the proper image cannot be obtained. In the type as shown in FIG. 11, when the original-through scanning is effected, since after the sheet original is once fed onto the glass platen, the sheet original must be shifted to the direction A meanwhile the image on the sheet original is read, the original exchange time is required to feed the sheet original to the platen without fail. In order to eliminate this long exchange time, as mentioned above, the higher speed sheet original feeding must be adopted, thus causing the aforementioned problems.
Further, in the apparatus shown in FIG. 10, a belt conveyor 7 is rotated in a direction E to feed the sheet original P onto the platen 3 and then is rotated reversely to return the sheet original to a direction D, thereby performing the switch-back feeding. Thus, a motor for driving the belt conveyor must be rotated normally and then reversely for a short time. However, generally, a motor has a building-up time for achieving a predetermined number of revolutions. If the building-up time of the motor is long, when the sheet original is read, discrepancy in a leading end portion of the image will occur during the reading of the sheet original. Further, with this arrangement, while the sheet original is being fed and read, the sheet original are stocked between inversion rollers 10, 12, with the result that the load to the belt drive motor is varied, thus causing the discrepancy in the image. In order to avoid such drawbacks, the image reading position of the image forming system may be shifted to a position 214 shown by the broken line in FIG. 11 not to reach the sheet original to the inversion rollers 10, 12 until the trailing end of the sheet original P is read; however, if do so, after the sheet original P is fed to the broken line position 215, the image must be read while switch-backing the sheet original, thus worsening the function of the image forming system considerably.